The present invention relates to a method for controlling an injection molding machine and, more specifically, a controlling method that is suitable to reduce variations in weight of molded products.
Referring now to FIG. 1, a motor-driven injection molding machine will now be described, centering on an injection unit included therein. The motor-driven injection molding machine comprises an injection unit driven by a servomotor. In such an injection unit, rotation of the servomotor is converted into a linear motion by a ball screw and a nut, thereby moving a screw forward and backward.
In FIG. 1, rotation of an injection servomotor 11 is transmitted to a ball screw 12. A nut 13 is fixed on a pressure plate 14 and moved forward and backward by rotation of the ball screw 12. The pressure plate 14 is movable along four guide bars 15, 16 (only two of them are shown in the figure) fixed on a base frame (not shown). Forward and backward motion of the pressure plate 14 is transmitted to a screw 20 via a bearing 17, a load cell 18, and an injection shaft 19. The screw 20 is rotatably and axially movably disposed in a heating cylinder 21. The heating cylinder 21 includes a hopper 22 for feeding a resin at the position corresponding to the rear portion of the screw 20. Rotation motion of a servomotor 24 for rotating the screw 20 is transmitted to the injection shaft 19 via connecting members 23 such as a belt or pulleys. In other words, the servomotor 24 rotates the injection shaft 19, which rotates the screw 20.
In a plasticizing/measuring process, the screw 20 rotates and moves backward in the heating cylinder 21, whereby a molten resin is stored in front of the screw 20, that is, in the heating cylinder 21 on the side of a nozzle 21-1. The backward movement of the screw 20 is effected by a pressure caused by the gradually increasing amount of a molten resin stored in front of the screw 20.
In a filling and injecting process, forward movement of the screw 20 is effected by driving force of the injection servomotor 11, whereby the molten resin stored in front of the screw 20 is filled and pressurized in a metal mold for molding. In this case, the force to press the molten resin is detected by the load cell 18 as an injection pressure. The detected injection pressure is amplified by a load cell amplifier 25 and fed into a controller 26. The pressure plate 14 is provided with a position detector 27 for detecting the amount of movement of the screw 20. The detected signal outputted from the position detector 27 is amplified by a position detector amplifier 28 and fed into the controller 26.
The controller 26 outputs current (torque) instruction values corresponding to the respective processes based on the set values preset by a display/setting unit 33 via a man-machine controller 34. A drive 29 controls current for driving the injection servomotor 11 to control output torque of the injection servomotor 11. A drive 30 controls current for driving the servomotor 24 to control the number of revolutions of the servomotor 24. The injection servomotor 11 and the servomotor 24 comprise encoders 31 and 32, respectively, for detecting the numbers of revolutions. The numbers of revolutions detected by the encoders 31 and 32 are fed to the controller 26. Especially, the number of revolutions detected by the encoder 32 is used to know the number of revolutions of the screw 20.
On the other hand, a plurality of heaters 40 are disposed around the heating cylinder 21 for heating and melting the resin fed from the hopper 22. These heaters 40 are controlled by a temperature controller 41. The temperature controller 41 receives the temperature detecting signals fed from a plurality of thermocouples 42 disposed in the vicinity of the heaters 40. The temperature controller 41 outputs the temperature detecting signals from the plurality of thermocouples 42 to the controller 26 as thermocouple-detected values. The temperature controller 41 also controls the plurality of heaters 40 based on the heater temperature-setting signals that represent the heater temperature setting values sent from the controller 26.
Actually, as shown in FIG. 2, multiple zones are defined around the heating cylinder 21, and the respective heaters are disposed in their respective zones around the heating cylinder 21 and independently controlled in terms of energization. Normally, multiple zones are defined in such a manner that a zone Z0 is allocated immediately below the hopper 22, and from there, zones Z1, Z2, Z3, Z4, and Z5 are allocated in sequence toward the nozzle 21-1.
In the injection molding machine, it is important to manufacture a large volume of products that are stable in quality in a short time at a low cost. Hereinafter, the description about the stable quality will be made limiting to the weight of the molded product. The controlling methods for obtaining a stable quality are as follows. The first method is a controlling method that can make correction for disturbances. In other words, feedback control maintains a characteristic that is considered to be an alternative to variations in weight of molded products in constant. The second method is a control method that aims at no-variation in weight by estimating variations in weight of the molded products in advance, and applying signals that cancels the estimated variations (feed forward control).
However, in the second controlling method, it is very difficult to ascertain the controlled object. Therefore, before using the second controlling method generally, many problems must be solved.
Referring now to the block diagram of FIG. 3, the outline of a mold internal pressure feed forward controlling method based on the second controlling method presented in the injection process will be described. In FIG. 3, Gc(S) represents a transfer function in the controller for controlling the injection servomotor 11 described in conjunction with FIG. 1, and Gp(S) represents a transfer function of the process. G1p(S) represents a transfer function for converting a disturbance such as variations in temperature of the heating cylinder 21 into variations in density of the molten resin. It is because variations in density of the molten resin effect on the mold internal pressure, and consequently, the weight of the molded product may vary. The disturbance is caused by various factors, for example, by variations in temperature of the heating cylinder 21, or by the state of the molten resin such as the temperature or the pressure, and the number of revolutions of the screw. In any cases, respective sensors that correspond to the respective types of the disturbance may detect such disturbances, and the detected results are fed to a subtracter 51. Assuming that the signal between the transfer function Gc(S) in the controller and the transfer function Gp(S) of the process is a value detected by the load cell described in conjunction with FIG. 1,
Gc (S)=value detected at the load cell (S)/disturbance (S),
Gp (S)=mold internal pressure/value detected at the load cell (S), are satisfied.
On the other hand, the transfer function G1p(S) is used for generating signals for canceling variations in amount of control caused by disturbances (in this case, the mold internal pressure that may effect on the weight of the molded product). Assuming that the amount of change in the mold internal pressure caused by the disturbance is xcex94p (t), the transfer function G1p(S) is used for generating the signal corresponding to xe2x88x92xcex94p(t).
As described above, in the current feed forward controlling method, variations in weight of the molded product is intended to be eliminated by maintaining the mold internal pressure at a intended value by detecting the disturbances that have been converted into variations in density of the molten resin and applying operational signals that can cancel detected variations in density of molten resin to the control system. The operational signals described here mean, specifically, signals of which the real stroke of the screw in the injection process is considered to be the amount of operation.
However, it is not easy to convert variations in density of the molten resin into the real stroke of the screw. Even for the identical variations in density of the molten resin, when changes occurred in the temperature of the resin or the amount of cushion at the time of injecting motion, the transfer function G1p (S) must be changed. This is difficulty of the feed forward control.
Difficulty of feed forward control will now be described from the different point of view. The primary cause of variations in density of the molten resin consists in variations in size of a resin material (variations in size of the pellet or ground material). In the actual molding operation, a method for stabilizing the molten resin density employed here uses the technique of changing the set temperatures for the zones Z1 and Z2 of the heating cylinder 21 shown in FIG. 2. More specifically, the set values of temperature for the zones Z1 and Z2 of the heating cylinder 21 are increased in the molding operation in which melting of a resin takes longer time than usual due to the reasons such that the size of a resin material is large, the molding cycle is short, or the measuring stroke is large. The reason is that melting temperature of a resin is not affected largely by a small change of the set values of temperature for the zones Z1 and Z2. Changing the number of revolution of the screw or the backing pressure of the screw may be contemplated as an alternative method, but this method cannot be employed easily due to its significant effect on the temperature of resin.
Accordingly, it is an object of the present invention to overcome the problems found in the feed forward control, and to provide a method for controlling an injection molding machine that can reduce variations in weight of the molded products.
It is another object of the present invention to stabilize the density of the molten resin and to reduce variations in weight of the molded product by adding an auxiliary feedback control system that considers the temperature of a heating cylinder as the amount of the operation.
The present invention is a method for controlling an injection molding machine. The method according to a first aspect of the present invention comprises the steps of measuring a density of a molten resin in a heating cylinder, controlling a stroke of an injection screw in an injection process by feed forward control based on the measuring step, and controlling an operating parameter of the injection molding machine based upon the measuring step.
The method according to a second aspect of the present invention comprises the steps of measuring a density of molten resin in a heating cylinder, determining a state of a temperature of the heating cylinder based upon a predetermined algorithm, and controlling a temperature of the heating cylinder based upon the measuring step and the determined state of the temperature, thereby minimizing variations in density of the molten resin.